This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of surfactants containing both anionic and nonionic polar groups molecularly linked to a common lipophilic base.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the introduction of water into the reservoir through an injection system comprised of one or more wells. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. Thus far, most low tension waterflooding applications have employed anionic surfactants. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, February 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One limitation encountered in waterflooding with certain anionic surfactants such as the petroleum sulfonates is the tendency of the surfactants to precipitate from solution in the presence of even moderate concentrations of divalent metal ions such as calcium and magnesium ions. Another limitation imposed upon the use of such anionic surface-active agents resides in the fact that desired low interfacial tensions can seldom be achieved, even in the absence of divalent metal ions, at salinities significantly in excess of 2 or 3 weight percent.
In view of these limitations, it has been proposed carry out waterflooding employing mixtures of anionic surfactants which will tolerate relatively high salinities and concentrations of divalent metal ions. U.S. Pat. No. 3,508,612 to Reisberg et al. is directed to a low tension waterflooding process employing a mixture of anionic surfactants which can be employed in saline solutions containing from 0.01 to 5 molar NaCl and from about 0 to 0.1 molar CaCl.sub.2. One of the anionic surfactants employed in the Reisberg et al. process is an organic sulfonate such as a petroleum sulfonate having an average molecular weight within the range of 430-470 and the other surfactant is a sulfated ethoxylated alcohol. A preferred sulfated alochol is one containing a C.sub.12 -C.sub.15 alkyl group and three ethylene oxide groups.
Another technique involving the use of a calcium-compatible mixture of anionic surfactants in low tension waterflooding is disclosed in U.S. Pat. No. 3,827,497 to Dycus et al. In this process, a three-component or two-component surfactant system may be employed. The three-component system comprises an organic sulfonate surfactant such as a petroleum sulfonate, a polyalkylene glycol alkyl ether, and a salt of a sulfonated or sulfated oxyalkylated alcohol. The two-component system comprises an organic sulfonate surfactant and a salt of a sulfonated oxyalkylated alcohol. These surfactant systems may be employed in a brine solution which, as noted in column 3, will usually contain about 0.5-8 percent sodium chloride and will often contain 50-5,000 ppm polyvalent metal ions such as calcium and/or magnesium ions. The sulfated or sulfonated oxyalkylated alcohols may be derived from aliphatic alcohols of 8-20 carbon atoms or from alkyl phenols containing 5-20 carbon atoms per alkyl group. The oxyalkyl moiety in this surfactant will usually be derived from ethylene oxide although other lower alkylene oxides containing 2-6 carbon atoms or mixtures thereof may be employed.
A number of recent patents are directed to the use of mixtures of anionic and nonionic surfactants in low tension waterfloods carried out in the presence of high divalent metal ion concentrations. For example, U.S. Pat. No. 3,811,505 to Flournoy et al. discloses a mixture of anionic and nonionic surfactants for use in formations containing water having concentrations of divalent ions such as calcium and magnesium within a range of about 500 to about 9,000 parts per million. The nonionic surfactants employed in the Flournoy et al. process include polyethoxylated alkyl phenols in which the alkyl group has from 5-20 carbon atoms and polyethoxylated aliphatic alcohols having from 5-20 carbon atoms. These surfactants are said to contain from 6-20 ethylene oxide groups. The anionic surfactants employed include alkyl sulfonates and phosphates having from 5-25 carbon atoms and alkylaryl sulfonates and phosphates having from 5-25 carbon atoms in the alkyl groups. Both the anionic and nonionic surfactants may be employed in concentrations within the range of 0.05 to 5.0 percent with the ratio of anionic surfactant to nonionic surfactant being about 0.1 to about 10.
U.S. Pat. No. 3,811,504, also to Flournoy et al., is directed to a low tension waterflood process for use in environments exhibiting a polyvalent ion concentration of about 1,500 to about 12,000 parts per million and which employs a three-component surfactant system containing two anionic surfactants and one nonionic surfactant. One of the anionic surfactants is an alkyl or alkylaryl sulfonate and the other anionic surfactant is an alkyl polyethoxy sulfate. The nonionic surfactant may be a polyethoxylated alkyl phenol or a polyethoxylated aliphatic alcohol as disclosed in the previously mentioned Flournoy et al. patent or it may take the form of fatty acid dialkanolamide or a fatty acid monoalkanolamide in which the fatty acid contains from 5-20 carbon atoms. In this process as in the previously described Flournoy et al. patent, a thickening agent such as a polyacrylamide or polysaccharide may be added to the surfactant slug or to a subsequently injected slug. In addition the surfactant slug may be preceded by a sacrificial agents such as sodium polyphosphate or sodium carbonate.